I. Technical Field
The present invention relates to centrifugal compressors and, more particularly, to an improved diffuser structure for such compressors which permits efficient compressor operation over a wide range of flow rates.
II. Discussion of the Prior Art
When centrifugal gas compressors are operated at less than full capacity, there is a tendency to produce surges in the rate of flow through the compressor. It is known in the prior art to employ positionally adjustable diffuser vanes, for different loading conditions, to vary the width of the flow passages defined between the vanes. Examples of positionally adjustable diffuser vanes of this type may be found in the following U.S. Pat. Nos. 908,227 (Elling); 916,156 (Huguenin); 2,392,200 (Thompson); 2,645,410 (Bauger et al); and 3,799,694 (Duzan). When such diffuser vanes are employed, gas flow between the vanes emerges as separate high velocity streams spaced by relatively low flow inactive regions. The interaction between the high velocity streams on the one hand and the relatively inactive gas on the other hand results in considerable turbulence. This turbulence significantly reduces operating efficiency and produces undesirable noise, particularly at low capacity positional settings of the vanes. Certain prior art attempts to eliminate the pockets of inactive gas between egressing streams involve the use of wedge-shaped vanes, wherein the thick trailing vane edges occupy spaces otherwise filled by inactive gas pockets. While this approach has achieved some success in eliminating turbulence, individual gaseous streams nevertheless pass into the discharge region. This tends to be less efficient than when the discharge region receives completely diffused outflowing gas.
Another problem concerning positionally adjustable vane diffusers in prior art centrifugal compressors is found at low outflow rate settings. In order to prevent uncontrolled surging, it is desirable that passages between overlapping vanes be reduced in size such that a high velocity flow of gas is maintained even though the mass flow rate of the gas is decreased. In conventional centrifugal compressors there is no provision for maintaining the high gas velocity at reduced mass flow rates, and therefore uncontrolled surges often result.
Some prior art centrifugal compressors, in order to effect positional adjustment of the vanes, attempt to pivot the vanes about respective axes which pass through the leading edges of the vanes. More particularly, if each vane pivots about its leading edge, the leading edge remains at a constant distance from the impeller throughout the entire range of vane movement. However, no prior art device has been able to achieve precise coincidence of the pivot axes and the vane leading edges, and the axes of the load bearing shafts supporting the movable vanes. Therefore, the leading edges of the vanes in most prior art compressors move radially to some extent when the vanes are pivoted.
Finally, it is desirable to provide a fully closed position of the adjustable vanes, wherein substantially no gas is permitted to flow from the impeller, past the vanes, to the outflow or discharge region. In such a mode, the impeller can be driven but, without a flow path for the gas, there is no kinetic load on the compressor; rather, the impeller merely recirculates gas in an annular path at the impeller periphery. This mode is ideal for minimizing start-up power requirements and also allows the compressor to run indefinitely in an unloaded standby condition. However, many positionally adjustable vane diffusers are not capable of effecting a substantial closure of the flow path between the impeller and the diffuser discharge outlet. Those that can present an uneven surface in the annular re-circulation region which creates considerable drag on the recirculating flow and a resulting load on the impeller.